Your M103 motor is fairly bulletproof-your symptoms surprise me. It certainly sounds like piston ring problems, but I suppose a top end (ie bad valve guide) could cause these symptoms.
Do you have the service history? Has the head ever been off?
I think the first thing I'd do is a compression test to see where you are, or perhaps better diagnose what ails your car
When properly maintained, you now own one helluva car-I hope you get it right and keep it that way!